Aircraft use radar altimeter systems or radar systems to determine distance to objects and the ground. One issue that every radar system must deal with is noise. In particular varying thermal noise inherent in radio receiving systems. To deal with the noise, some radar systems use a varying threshold. Any signal below the threshold is discarded as noise and any signal above a threshold is considered a valid target. Setting the threshold is critical because if it is set too high the radar system may not track valid weak signal returns and if it set to low the system could track invalid targets due to noise. A typical method of setting the threshold is by using a determined signal to noise ratio (noise ratio). In this method, the noise is monitored through a noise channel. Gain in a receiver stage of the system is used to keep the system at the select noise ratio. Gain is controlled by a noise automatic gain control (NAGC) circuit. This can generally be referred to as a gain control loop or the control loop. In this type of system, with an increase of noise, the system via the gain control loop will reduce the receiver gain to keep a constant noise ratio.
One common problem encountered with the use of the noise ratio to control the gain is that noise spikes can be seen by the system as thermal noise. As a result of a noise spike, the gain control loop reduces the receiver gain to maintain the constant noise ratio which will result in a loss of system sensitivity. This can lead to the radar system losing the track of a ground return or other valid radar return. Noise spikes can occur when two radar systems are in close proximity to each other while their transmitters are transmitting in the same frequency band. For example, when two aircraft are in wingman formation. These noise spikes are caused by both cross coupling that occurs directly between the airplanes and the returns off the ground that originally were transmitted from another altimeter. The effects of cross coupling and detecting another altimeter's return can be reduced by periodically changing frequency or the pulse repetition interval. However, random spikes will still occur.
For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an effective and efficient method and apparatus for dealing with noise spikes.